The invention relates to a process for the separation of one or more olefinic or aromatic unsaturated compounds from a liquid, paraffin-rich hydrocarbon mixture. The invention enables unsaturated compounds to be almost completely separated from liquid hydrocarbon mixtures whose components have only slight physical and chemical differences from each other. The invention is particularly, but not exclusively, applicable to the separation of aromatic hydrocarbons from hydrocarbon mixtures which consist almost wholly of paraffin.
The separation of unsaturated compounds from paraffin-rich hydrocarbon mixtures is known and has been carried out for a long time by treating the crude hydrocarbon mixture with a liquid, such as concentrated sulphuric acid or a selective solvent, such as liquid sulfur dioxide or furfural. To achieve almost complete separation of the unsaturated compounds, however, the known refining processes involve considerable outlay. For economic reasons these processes are therefore usually only used for the primary purification of crude hydrocarbon mixtures.
Liquid hydrocarbon mixtures which contain only small amounts of unsaturated compounds can be further purified in an economic way, as is well known, by treating with solid adsorption agents. Thus, for example, the German Offenlegungsschrift P 22 52 305.3 discloses a process for the production of purified n-paraffins in which a technically accessible n-paraffin mixture is freed of carcinogenic polycyclic aromatic hydrocarbons by treatment with a certain aluminum-oxide adsorption agent to such an extent that the purified n-paraffins can be used for the production of biogenic proteins for use in foodstuffs. In this process the crude n-paraffins mixtures are passed through a fixed-bed of the adsorption agent. However, it is difficult and costly to completely free the adsorption agent again from the aromatic compounds bound therein and to regenerate it for renewed use.
U.S. Pat. No. 3,409,691 relates to another process wherein the separation of mixtures containing substances of different polarity is performed using dehydrated macroporous cation exchange resins in the form of their metal salts as selective adsorption agents. However this U.S. patent makes as little mention of economic regeneration of the used exchange resin as it does of separation, in particular of polycyclic aromatic compounds which are contained in only small amounts in a starting material substantially consisting of higher paraffins.
East German Pat. No. 62 385 discloses a process by means of which polycyclic aromatics and other contaminating substances can be removed from crude oil paraffins (slack wax) by adsorption on zeolitic molecular sieves.
Our U.S. Pat. No. 3,960,703 (equivalent to German Offenlegungsschrift No. 23 64 333) describes a process for the purification of crude oil paraffin (slack wax) in which the liquified starting material is treated with a dehydrated macroporous ion exchange resin which is charged almost completely with metal ions, in particular silver or copper ions, and which is expediently arranged as a fixed-bed in a column. This process supplies a product which fulfills the purity requirements of the German Manual for Pharmacists and Pharmaceutical Industry (DAB VII-Deutsche Arzneibuch VII) and which is therefore suitable for biogenic protein synthesis, for pharmaceutical and cosmetic purposes as well as for use in the food industry. However, the regeneration of the ion exchange resin used for separation is relatively involved, since the ion exchange resin must be rinsed with an inert hydrocarbon, thereafter treated with an alcohol, ether or ketone in order to desorb the separated aromatics and finally rinsed again with an inert hydrocarbon and dried.
Our U.S. Pat. No. 3,979,280 (equivalent to German Auslegeschrift No. 23 64 306) describes and claims a process for the separation of unsaturated olefinic or aromatic compounds from liquid hydrocarbon mixtures containing them wherein the physical-chemical properties of the components of the mixture differ only slightly or wherein only traces of the unsaturated compounds occur in the mixture, which comprises adsorbing the unsaturated compounds on a macroporous, dehydrated cation exchange resin laden with a metal ion and having a specific surface area of at leat 1 m.sup.2 /gram and a pore diameter greater than 10 A, and, subsequently, displacing the unsaturated compounds using a normally gaseous hydrocarbon.
The process of U.S. Pat. No. 3,979,280 simplifies the process of U.S. Pat. No. 3,960,703 in that the unsaturated compounds of the ion exchange resin are desorbed and displaced in U.S. Pat. No. 3,979,280 at temperature of 10.degree. to 40.degree. C. and pressures of 1 to 30 atmospheres by employing a normally gaseous hydrocarbon in the liquid phase as the desorption agent. Liquified propene or butene are disclosed as preferred desorption agents. In this process the exchange resin used for separating unsaturated compounds is rinsed once only with an inert hydrocarbon and is thereafter desorbed with the liquified olefin. The desorption agent is then allowed to evaporate. A second rinsing of the exchange resin can be dispensed with in this way. However, a disadvantage of this process is that this process must be carried out at excess pressure and the liquified olefins used for desorption sometimes tend to form polymers, which can impair the regenerability of the exchange resin.
It is advantageous to further simplify and improve processes for separating unsaturated compounds, in particular polycyclic aromatic hydrocarbons, from paraffin-rich hydrocarbon mixtures by treating with dehydrated macroporous cation exchange resins in form of their metal salts.